The present invention relates to a method and apparatus for preventing the surface of a sample from being damaged due to ion beam irradiation upon obtaining a sample observation image using a focused ion beam (FIB), and more particularly to an observation method and processing method suited for repairing a semiconductor photomask and an apparatus therefore.
The existence of the scanning ion microscope (SIM) is widely known together with the electron microscope as a microscope with high magnification. Although principally analogous to a scanning electron microscope (SEM), there are great differences in that, instead of electrons, ions are irradiated as a beam onto a surface of the sample and secondary charged particles emitted from the surface of the sample are not limited to electrons but include ions. Due to the difference in the emitted secondary charged particles, the scanning image based on the detected particles (SEM image and SIM image) differs in resolution sensitivity depending on the sample material. Accordingly, an ion beam microscope is selected where a SIM image is more distinct for the kind of a sample to be examined than a SEM image. In order to enable comparison and consideration of both types of scanning images, and thereby enabling more detailed observation, both microscope images are often obtained.
The focused ion beam apparatus has been put into use in semiconductor-related fields for ten-plus years, and its history is not extensive. However, the recent advancement in ion optical system basic performance capabilities is conspicuous, and it has become possible to obtain a high degree brightness and high resolving power apparatuses for use as observation microscopes and high accuracy apparatuses as processing apparatuses. First, the essentials of the scanning ion microscope apparatus, which is a prerequisite technology for the present invention, will be explained using FIG. 4 as reference. Reference numeral 1 is an ion source (e.g. Ga liquid metal), 2 is an ion beam, 3 is an electrostatic optical system formed by a capacitor lens 31, beam blanker 32, aligner 33, movable restriction 34, stigmator/aligner 35, objective lens 36 and deflector 37, 5 is a gas injector, 6 is a secondary charged particle detector, and 7 is a sample.
When a focused ion beam apparatus is used to conduct processing with high accuracy, it is necessary to specify the processing point in advance of processing or to take a SIM image in order to accurately define the processing area. This requires irradiation by an ion beam to the sample. The ion beam irradiation causes unintentional damage to the sample which has been a problem. Conventionally, to prevent such damage the only effort made has been to restrain ion beam irradiation amount to the greatest possible extent. No measure has been taken to solve the crux of the problem.
As stated before, in the case of a scanning ion microscope, the sample surface being damaged by ion beam scanning conducted for obtaining a microscope image is a problem. That is, as opposed to the electron beam of an electron microscope, the ion microscope uses charged particles of larger mass to create an ion beam directed at the sample surface and hence there is a far greater momentum at the time of bombardment than in the case of electrons. Due to this, there are problems such as damage to the sample surface by the sputtering phenomenon, change in the sample material caused by ion implantation into the sample, and so on. The purpose of the present invention is to provide a method that does not damage a sample when it is irradiated by an ion beam to obtain a SIM image, and an apparatus to carry out the inventive method.
In particular, a correction method and an apparatus therefor is provided which does not cause damage to the underlying substrate or the pattern due to ion beam irradiation in order to obtain a SIM image conducted prior to processing when conducting defect correction of a semiconductor photo mask by using an ion beam.